Lecture 7: Enzymes are Essential for Life (Bioc 192)

Course Context and Materials

• Course materials (slides, booklets, learning resources) are copyrighted and may be used only for the University’s educational purposes. Private study or research is allowed, but no copies or distribution beyond the course context.
• Slide references for Lecture 7 – Enzymes are Essential for Life: Part 1 – Enzymes are Essential for Life, pages 27–31; Textbook references include various chapters in Campbell et al., Snape et al., Voet & Voet, etc. (as listed in the transcript).
• Social media handles for the course: Twitter @otagobiochemist; Facebook Biochemistry.Otago.

Inputs and Outputs of Life (Metabolic Perspective)

• INPUTS to a living system include:
  • Food, Water, Energy, Oxygen, Signals
• OUTPUTS of life include:
  • Work, CO₂, Offspring, Water, Waste
• Concept: Life is a large network of chemical reactions with a purpose-built architecture that sustains non-equilibrium conditions to do work.
• Illustration note: A schematic shows a system with inputs → chemistry → outputs, highlighting that life maintains a highly organized network rather than static equilibrium.

Learning Objectives for This Lecture

• Understand and describe:
  • The general roles of enzymes in biology
  • Thermodynamic features of an enzyme-catalyzed reaction
  • The roles of cofactors in enzyme-catalyzed reactions
  • The fact that individual enzymes do not act in isolation (networks matter)

Life is NOT at Equilibrium: Thermodynamics Basics

• For any chemical/biological process, Gibbs free energy change is key: $\Delta G < 0$ (spontaneous; energy released), $\Delta G > 0$ (non-spontaneous; energy required), $\Delta G = 0$ (at equilibrium; cannot do work).
• Life operates at a steady state, not at equilibrium (Campbell reference).
• Important distinction: Spontaneous does not imply fast. Spontaneity refers to energy release, not reaction rate.
• In metabolic networks, most steps are kept away from equilibrium to enable continuous throughput and work.

Metabolic Coupling: Keeping the Network Non-Equilibrium

• Enzymes can couple a spontaneous reaction to a non-spontaneous one, enabling an overall Gibbs free energy change \Delta G_{net} < 0 even if one step is unfavorable.
• This coupling is a fundamental feature of metabolism; it helps drive energetically uphill processes by harnessing downhill steps.
• Diagrammatic note: The network shows cycles and branching where coupling ensures net negative \Delta G)** for the pathway.

Activation Energy and Transition States: Getting into the Timescale for Life

• Reactions pass through high-energy transition states; activation energy E_a is required to reach the transition state.
• Free energy profile concept:
  • Reactants → Transition state → Products
  • Transition state represents the energy barrier that must be overcome.
• Enzymes act by lowering the activation energy, thus accelerating the reaction rate.

Enzymes and Catalysis: How They Make Reactions Happen Faster

• Enzymes catalyze thermodynamically favorable reactions by lowering the activation energy (Ea).
• Consequence: Enzymes speed up both forward and reverse reactions equally; the overall equilibrium constant (related to \Delta G^{\circ}) is not changed by the enzyme.
• Structural note: Most enzymes are proteins; some RNA enzymes (ribozymes) exist.
• Example highlighted: Glycogen phosphorylase catalyzes the breakdown of glycogen by cleaving α-1,4-glycosidic bonds to produce glucose-1-phosphate.

Enzyme Classes (Six Major Classes)

• 1) Oxidoreductases — redox reactions (transfer of electrons)
• 2) Transferases — transfer of functional groups
• 3) Hydrolases — hydrolysis reactions (use of H₂O)
• 4) Lyases — non-hydrolytic breaking or making of bonds
• 5) Isomerases — transfer within a molecule to yield an isomer
• 6) Ligases — join two molecules together (often coupled to ATP cleavage)

Cofactors: Helpers for Enzyme Catalysis

• Many enzymes require non-protein factors (cofactors) to catalyze reactions.
• Two major classes:
  • Metal ions
  • Coenzymes

Metal Ions as Cofactors

• Metal ions act as Lewis acids (electron acceptors) and participate in acid-base catalysis.
• They help position substrates precisely via coordination chemistry.
• Common examples and associated enzymes:
  • Mg²⁺, Zn²⁺: DNA polymerase; hexokinase; pyruvate kinase
  • Fe²⁺/Fe³⁺: Cytochrome oxidase; peroxidase
• These ions enable precise geometry and stabilisation of transition states within active sites.

Coenzymes: Small Organic Molecules that Carry Groups or Electrons

• Coenzymes are small organic molecules that act as carriers (electrons, atoms, or functional groups).
• They are often derived from vitamins.
• Examples of coenzymes and their vitamin precursors (from a Campbell/Otago table):
  • Biotin — carboxylation reactions
  • Pantothenic acid — part of Coenzyme A (acyl transfer)
  • Riboflavin (vitamin B2) — flavin cofactors (oxidation-reduction)
  • Niacin — nicotinamide adenine dinucleotide (NAD⁺/NADP⁺; oxidation-reduction)
  • Pyridoxal phosphate (PLP) — transamination and amino-group chemistry
  • Folic acid — one-carbon transfer chemistry
  • Thiamine (vitamin B1) — thiamine pyrophosphate (TPP; aldehyde transfer)
• Summary: Coenzymes shuttle electrons or functional groups and expand the catalytic repertoire of enzymes.

The PLP Cofactor and Glycogen Phosphorylase Mechanism (A Concrete Example)

• The PLP cofactor facilitates glycogen phosphorylase activity.
• PLP is covalently linked to a lysine residue in the enzyme active site.
• Mechanistic feature: PLP acts as a proton donor/acceptor, enabling cleavage and transfer steps during glycogen degradation.
• Conceptual schematic (from slide):
  • Terminal glucose unit becomes a carbohydrate cation intermediate (carbocation-like transition state)
  • PLP stabilizes intermediates and assists proton transfers to Pi (inorganic phosphate)
  • Active site returns to its original state after product formation
• Visual cues in the slide show: a covalent PLP-lysine linkage; carbocation-like intermediate; formation of glucose-1-phosphate as the product

The Metabolic Network: Keeping Most Steps Away from Equilibrium

• Diagrammatic note (Voet & Voet): a network including key metabolites such as Pyruvate, Acetyl-CoA, Glutamate, and Aspartate.
• Core principle: The metabolic network uses enzyme-catalyzed steps to keep most individual steps from approaching equilibrium, enabling controlled flux.
• Enzymes couple spontaneous reactions to non-spontaneous ones to yield an overall \Delta G_{net} < 0, driving metabolism forward.

Free Energy, Activation, and Enzymatic Rate Enhancements: A Quantitative Snapshot

• Conceptual: Uncatalyzed vs. Enzymatically catalyzed energy landscapes for a given reaction.
• Example: Hexokinase-catalyzed phosphorylation of glucose (glucose + Pi → glucose-6-phosphate + H₂O) vs the same transformation coupled to ATP hydrolysis (ATP + glucose → ADP + Glucose-6-P).
• Numerical anchors from the slides:
  • Uncatalyzed: \Delta G^{\circ} \approx +13.8\ \mathrm{kJ\,mol^{-1}} for Glucose + Pi → Glucose-6-P + H₂O
  • Coupled reaction (glucose + ATP → glucose-6-P + ADP): \Delta G \approx -30.5\ \mathrm{kJ\,mol^{-1}}
  • Follow-on or catalyzed step: \Delta G^{\circ} \approx -16.7\ \mathrm{kJ\,mol^{-1}} (for the subsequent step involving Glucose-6-P + ADP, per the slide)
• Consequences: Coupling ATP hydrolysis to substrate transformation makes the overall reaction highly favorable.
• The catalytic effect of the enzyme dramatically reduces the time required for the reaction to proceed.

Ground State Destabilization vs Transition State Stabilization (How ΔG is Lowered)

• Two general strategies to lower the activation barrier (to be covered in tomorrow’s lecture):
  • Ground state destabilization: Raise the energy of the reactants or destabilize the ground state
  • Transition state stabilization: Stabilize the transition state via active-site complementarity (shape and charge)
• Both strategies can be achieved by arranging the active site to better complement the transition state, rather than the substrate itself.

Key Concepts and Takeaways

• Life is a network of reactions that is not at equilibrium, enabling useful work to be done.
• Enzymes increase reaction rates by lowering the activation energy required to reach the transition state.
• Cofactors are often required for catalysis and are thus essential in the diet.
• Enzymes do not alter the overall equilibrium of a reaction, but they dramatically alter the rate at which equilibrium is approached.
• The metabolic network relies on reaction coupling to drive energetically unfavorable steps through favorable ones, yielding an overall negative \Delta G) for pathways.
• Cofactors fall into two broad classes: metal ions and coenzymes; each provides distinct catalytic capabilities.
• The PLP cofactor is a specific example of how a cofactor can facilitate a particular enzymatic step (glycogen phosphorylase activity).
• Vitamin-derived cofactors play critical roles across metabolism, highlighting the link between diet and enzymatic function.

Objective-Based Self-Assessment Prompts

• Review: What is the general role of enzymes?
• Why do enzymes catalyze biological reactions?
• Will a biological process occur spontaneously if \Delta G > 0? Why/why not?
• Do enzymes alter the equilibrium of a reaction?
• How do enzymes affect the thermodynamics of a reaction?
• What are the major classes of enzymes?
• What is a cofactor, and what purpose do they serve?
• What are the two major classes of cofactors?
• How do metal ions aid enzymes in metal-ion catalytic mechanisms? Illustrate with an example.
• What is a coenzyme and how is it derived?
• How do coenzymes aid in enzymatic reactions?
• What is reaction coupling and how do enzymes mediate this?

Notes on Concepts and Notation Used in the Slides

• Key thermodynamic quantities used:
  • Gibbs free energy change: $\Delta G$
  • Activation energy: $E_a$
  • Transition state: high-energy configuration along the reaction coordinate
  • Standard free energy change: $\Delta G^{\circ}$
• Conceptual relations:
  • Enzymes lower $E_a$ to accelerate both forward and reverse reactions without changing $\Delta G$ of the overall reaction
  • Coupling can convert an unfavorable step into a favorable one via an overall negative $\Delta G_{net}$
• Real-world relevance: Understanding enzyme action helps explain metabolic flux, energy transduction, and the nutritional importance of cofactors.

Quick Reference: Notation Summary

• Uncatalyzed reaction example: $\mathrm{Glucose} + \mathrm{Pi} \rightarrow \mathrm{Glucose\text{-}6\text{-}P} + \mathrm{H_2O}$ with $\Delta G^{\circ} \approx +13.8\ \mathrm{kJ\,mol^{-1}}$
• Coupled to ATP hydrolysis: $\mathrm{Glucose} + \mathrm{ATP} \rightarrow \mathrm{Glucose\text{-}6\text{-}P} + \mathrm{ADP}$ with $\Delta G \approx -30.5\ \mathrm{kJ\,mol^{-1}}$
• Subsequent catalytic step (for completeness): $\Delta G^{\circ} \approx -16.7\ \mathrm{kJ\,mol^{-1}}$
• Representative rate enhancements: from t1/2 values, enzyme catalysis can reduce millennia-long timescales to seconds (e.g., from ~10⁶–10⁷ years to ~1–10 s range), yielding rate enhancements on the order of $\sim 3 \times 10^{14}$.
• Example of a cofactor mechanism: PLP in glycogen phosphorylase involves covalent linkage to a lysine and facilitates proton transfer steps to Pi, stabilizing key intermediates.

End of Notes